The manufacture of turbine shafts by welding has been known for a long time. It includes the following steps:
the shaft components to be welded are machined so that they interlock while leaving a circular groove in between two adjacent components; these components are then stacked vertically;
the assembly thus formed is heated to a temperature of about 300.degree. C.;
the bottoms of the grooves are welded in several passes, here called bottom passes, and preferably under an argon atmosphere;
the shaft is placed in a horizontal position;
the grooves are filled by several passes of automatic welding; and
stress is released by annealing.
The quality of the bottom pass welding is inspected before the filling passes are performed.
This inspection is made by gamma radiography; for that purpose, a radioactive source which emits gamma rays is inserted inside the shaft and held on its axis.
A film which is sensitive to gamma rays is placed in the groove delimited by two adjacent shaft components welded together, and is exposed for a suitable period of time to the gamma radiation which passes through the weld. After development, the film shows up possible defects in the weld such as cracks, failure of the weld to penetrate, etc.
Such quality control by gamma radiography requires the welded parts to be cooled from the temperature of 300.degree. C. to which they are raised, as stated above, to satisfy the requirements of good welding technique. Indeed, a gamma radiography film cannot withstand a temperature of more than 40.degree. C. and the exposure times required to take a photograph range between 15 minutes and 11/2 hours depending on the degree of activity of the radioactive source.
Therefore, it is understandable that taking quality control photographs requires parts to be prior cooled from 300.degree. C. to a temperature close to ambient temperature.
Now, it takes several days to cool large parts such as, for example, the shaft of a steam turbine for a 1300 megawatt generator.
If, in extraordinary circumstances, a defective weld is detected, the part must be reheated, the defect repaired, the part cooled and a new photograph taken.
These operations take a considerable time (up to 10 days) during which the welding unit cannot be used. Now, the greater part of the time during which the welding unit is unavailable is taken up by the cooling time and since the cooling time cannot be reduced, techniques have been sought to reduce the time during which the welding unit is unavailable e.g. by performing the gamma radiography while the parts are still at a high temperature.
With this aim in view, film support cassettes have been produced which are cooled to keep the film at a bearable temperature while it is in the neighbourhood of a metal part which is brought down to a temperature of about 200.degree. C. or more.
Previously manufactured cassettes are not satisfactory. In some cases, where they are air-cooled, cooling is insufficient for the application in question. Other cassettes include a layer of water interposed between the film and the source of heat and radiation. Cooling is then sufficient but the quality of the photo is unsatisfactory because the layer of water produces parasitic images on the film.
One aim of the invention is to produce a cassette which does not have the above-mentioned disadvantages.